ECM advancements in the 1980s vastly improved the ability to optimize efficiency and performance and minimize emission concerns with continuous-flow spark-ignited internal combustion engines. By continuously monitoring numerous sensors and inputs, ECM's can balance the current operator commands against performance conditions to determine the most ideal fuel flowrate needed for the engine at any given instant.
Knowing the ideal flowrate and delivering it, however, are two very different things. Even though modern ECMs can know the ideal at any given instant, practical prior art fuel supplies are not able to consistently deliver it instantaneously on demand across their entire range of operation. The very best of available controls claim to provide 1% setpoint accuracy, which means they claim to deliver an actual fuel supply flowrate within about 1% of the demanded flowrate. The ability to consistently deliver a continuous-flow gaseous fuel flowrate with 1% setpoint accuracy is considered extremely accurate and would be ideal, but claims to that effect tend to only be part of the story.
With the prior art, extreme setpoint accuracies tend to only be attained within a limited range of operation, which means that claimed accuracies are generally unreliable, especially for engine's having large dynamic power ranges. (An engine's “dynamic power range” is the ratio of maximum power to minimum power over which the engine will operate as specified, which is dependent largely on the effective turndown ratio of the associated fuel supply system.) For a fuel supply delivering 25 grams/second at the top end of its operating range, for instance, one percent would be a quarter-gram/second (0.25 g/s). While calibrating one of the best available valves to a quarter-gram/second error can be manageable for moderate flowrates, the same fuel supply often needs to also idle at about a quarter-gram/second at the opposite end of its operating range, such that the same quarter-gram/second error would be tremendously inaccurate for near-idle flowrates. Although accurate control is sometimes considered easier to achieve with lower flowrates, 1% setpoint accuracy at a quarter-gram/second idle flowrate would require accuracy to within ±0.0025 g/s. So, while prior art gas flow valves claim to deliver extremely accurate flowrates at specified portions of their overall operating range (often at 200 kPa), it has long been unattainable to achieve as much for both ends of the operating range and everything in between, especially for such large ranges in real-world operation.
The complex interaction of too many real-world variables frustrates the pursuit of consistently-high, full-range setpoint accuracies for continuous-flow fuel supplies. Wear and tear, leaks, lag times, glitches, clogs, noise, artifacts, and general variability all tend to happen in the real world. External temperatures and wide variability in gaseous fuel compositions further compound the challenges.
Moreover, even if perfection was achievable within a fuel supply's flowrate control valve itself, flowrate accuracies can be thwarted by upstream and downstream pressure fluctuations as well. Because gaseous fuels are compressible, downstream events related to combustion or valve and piston movements can cause pressure waves that create sizable flowrate fluctuations. Upstream pressure fluctuations can be equally problematic, especially when controlling the flowrate of vaporized liquid fuels (e.g., LNG or LPG).
The control difficulties with vaporized liquids arise largely due to the dramatic variation in tank pressures during the course of operation. When an LNG fuel tank is medium full, the supply pressure generally remains manageable enough. However, control challenges increase when the tank is full because of the lack of instantaneous vapor pressure capacity, and also when the tank is closer to empty, as the amount of liquid fuel is vaporized over time. Controlling vaporized liquid fuels is all the more challenging if vaporization is less than complete at the source. If any residual liquid phase remains after passing through the vaporizer, such as too often occurs when heat exchangers become clogged, dramatic pressure spikes can arise within the flowrate control valve or at other locations that can frustrate the most reliable of control systems.
As a result, conventional gas flow valves do not consistently achieve flowrate setpoint accuracies that are fine enough to ensure optimal power and emissions over large dynamic power ranges. Even the best of controls are generally unable to consistently achieve and continuously maintain 1% flowrate setpoint accuracies across anything more than about a 15:1 or maybe 20:1 dynamic power range. Despite claims that might imply otherwise, most existing valves for large dynamic power ranges in practice generally only have “ballpark” accuracy (i.e., between 3% and 10% setpoint accuracy) over significant portions of their specified operating range. Although they produce their specified accuracy in certain easy portions of their operating range, such accuracies are usually limited to the middle or upper half of that operating range, providing overall fuel use and emission levels that fall far short of the idealized levels otherwise expected.
Thus, there has long been a need for an affordable, continuous-flow valve that can consistently and instantaneously deliver ECM-demanded mass-flowrates with extreme accuracy across a large dynamic power range, particularly in the field of vaporized natural gas fuel systems for spark-ignited engines.